This invention relates generally to methods and apparatus for testing core samples extracted from an oil or gas well. In particular, the invention relates to a method of determining elastic properties of a core sample and an apparatus useful in performing the method, which apparatus is for detecting changes in longitudinal and lateral dimensions of the core sample.
A commonly utilized technique for stimulating the production of hydrocarbons from a subterranean rock formation penetrated by a well bore is to create and extend fractures in the formation. Generally, the fractures are created by applying hydraulic pressure on the formation from the well bore. That is, a fracturing fluid is pumped through the well bore and into the formation at a rate and pressure such that the resultant hydraulic force exerted on the formation causes one or more fractures to be created. The fractures are extended by continued pumping; and the fractures can be propped open or flow channels can be etched in the faces of the fractures with acid, or both can be done, to provide openings in the formation through which hydrocarbons readily flow to the well bore. Fracturing is also utilized in carrying out enhanced production procedures in subterranean formations (e.g., water flooding from an injection well to a production well) as well as in other applications.
In designing fracturing treatments to be carried out in subterranean rock formations, it is often necessary and always desirable to know the direction in which fractures will extend in the formation. Such knowledge enables more efficient reservoir management. For example, knowing such directional information allows one to better place production wells for maximizing production from the reservoir of hydrocarbons in the subterranean formation and to better place waterflood injection wells for increasing waterflood sweep efficiency by avoiding an injection well arrangement that would cause premature breakthrough of the injected fluid into the producing well.
Information that can be used to help predict fracture direction includes Young's modulus and Poisson's ratio, which describe elastic properties of rock. These can be determined by testing rock core samples that have been extracted from an oil or gas well in a known manner.
Young's modulus can be defined as the ratio of normal stress to the resulting strain in the direction of the applied stress. Stress can be applied to a core sample with a longitudinal or axial compressive force from a known type of press. The resulting longitudinal or axial strain is the yield or deflection measurable as the change in the longitudinal or axial dimension of the core sample.
Poisson's ratio can be defined as the ratio of lateral or radial strain to the longitudinal or axial strain for normal stress within the elastic limit. This is measurable using the aforementioned detected dimensional change in conjunction with a lateral or radial dimensional change detected in response to the applied stress.
There are prior types of core sample test methods and apparatus with which to apply forces to core samples and measure responses to such forces; however, we are not aware of any in which core samples can be tested and removed without removing a sealing sleeve from inside a supporting housing of a test instrument and in which both longitudinal yield and lateral yield can be concurrently sensed and in which such sensing can be done under temperatures up to about 300.degree. F. and radial and pore pressures up to about 10,000 psi to better simulate actual downhole conditions and thereby provide more accurate indications of formation properties. To facilitate testing and to provide more useful information, there is the need for an improved method and apparatus for testing core samples which has the aforementioned features we do not find in any one prior test method or apparatus of which we are aware.